This comes from: http://www.filesuffix.com/extension/idx.html
"IDX file is a LaTeX Unsorted Index Data. It is an makeindex input file. LaTeX is a high-quality typesetting system, with features designed for the production of technical and scientific documentation. LaTeX is the de facto standard for the communication and publication of scientific documents."
At the bottom of the page it says:
"Open OPX Files
Open OPX Files with OrgPlus. Download Free Trial."
This takes you to http://www.orgplus.com/download/organizational-chart.htm?Campaign_Id=1421&Activity_Id=261&_kk=opx%20file%20extension&_kt=85c5b68d-87f7-4657-a832-a22ec6b7c74b&gclid=CJ-KktG84pMCFRMYagodmCjbVw